This invention relates to the mounting of semiconductor die and more specifically relates to a novel process for the manufacture of semiconductor die with an integral insulation underfill.
When semiconductor die such as flip chip die having solder ball connectors on one surface thereof are soldered to a support board, an insulation underfill is commonly provided to fill the volume bounded between the bottom of the die and the top of the board and between the solder balls to improve the device temperature cycling capability. This process usually requires an extra step by the user to deposit the insulation underfill. Commonly, the user will conventionally solder the device solder balls to conductive traces on a printed circuit board and then deposit a low viscosity epoxy material around the perimeter of the die, relying on capillary action to uniformly draw the epoxy while it is still liquid into the interior spaces between the die bottom and printed circuit board top surface.
It is also known to employ wafer level underfills in which the areas around all solder balls are flooded with an epoxy or a thermoplastic material which contains a solder flux. The wafer is then singulated with the plastic underfill in place for each die. During the solder down operation, the underfill material liquefies while reflowing the solder balls, releasing the flux to the solder and flowing around the solder balls. However, this process does not work well and is not in use.
It would be desirable to provide a novel wafer level underfill system for improving the solder down of semiconductor die, or any other product employing solder ball terminals.
In accordance with the invention, discrete spaced columns of an underfill plastic are deposited, in the wafer stage and prior to singulation, between solder balls and to a height equal to the final standoff height of the reflowed die. By standoff height is meant the final desired space between the bottom of the die and the top of the support surface receiving the die, such as a printed circuit board surface. The plastic is then engineered to become semifluid at the solder reflow temperature, attaching the die to the board through the plastic columns. Thus, the plastic columns, while not totally attaching the underside of the die to the circuit board, will add significant strength to the overall bond, reducing the direct load on the solder balls.
The underfill plastic of the invention may be a thermoplastic, or, preferably, a thermosetting epoxy. The plastic may be in pellet form. The plastic may be applied to the wafer by any desired screening or dispensing system and contains a solvent which makes the plastic material (which is solid at room temperature) fluid and compatible with dispensing or printing. Once deposited, the solvent will be driven off, leaving a solid epoxy, this occurring before the wafer or die are supplied to an end user. The die will be mounted by a user who mounts the device into a conventional solder paste used for connecting solder balls.
During solder reflow the plastic becomes at least semi-fluid at about 80xc2x0 C., forming the connection between the die and the PCB. The reason for this early phase change is that flux in conventional solder paste starts to become mobile at temperatures in excess of about 80xc2x0 C. and the epoxy should make its connection between die and PCB before the area becomes contaminated with flux residue. The epoxy will then begin its cure by raising the temperature to 150xc2x0 C. or more.
Several methods for depositing thermosetting epoxies can be employed.
In a first method, a screen with clearance cut outs which align with and fit over the solder balls align central openings with the areas to receive insulation pillars. A thermosetting material which is solid at room temperature is thinned with solvents and is printed onto the wafer substrate in a screen printing type process so that plastic columns are printed between solder balls. The solvent is then driven off in a low temperature bake in the oven, leaving solid pillars of thermosetting plastic between the solder balls.
In a second method, the thermosetting plastic can be thinned with a solvent so it can be conveniently dispensed from a syringe, depositing self supporting columns of plastic between solder balls. The solvent is then driven off in a low temperature step, leaving solid plastic columns.
In a third method, very shallow columns of thinned thermosetting plastic are first deposited between solder balls, and, very small diameter pellets of the solid plastic (small in relation to the diameter of a column) are then scattered, or dusted over the wafer surface, adhering mainly to the area coated with the solvent rich shallow dots of plastic. The pellets outside of these areas are removed, as by inversion of the wafer or by air flow over the surface, and the solvents are then driven off in a low temperature bake to complete the process.